1. Field of the Invention
The present invention relates to a differential-phase polarization-sensitive optical coherence tomography system that is capable of obtaining three tomographic images indicating a reflectivity, a phase retardation, and a fast axis angle of a specimen simultaneously.
2. Description of the Related Art
Phase retardation between P and S waves and the orientation angle of fast axis (hereinafter referred to as fast axis angle) are two parameters defining linear birefringence properties of anisotropic materials. K. Schoenenberger et al. (Applied Optics 37, pages 6026-6036, 1998) proposed the use of a circularly polarized laser beam in conventional polarization-sensitive optical coherence tomography (PSOCT) to obtain the phase retardation in terms of a ratio of demodulated amplitudes of P-polarized and S-polarized heterodyne signals. However, the fast axis angle is not available in their setup. C. Hitzenberger et al. (Optics Express 9, pages 780-790, 2001) proposed a method to calculate the fast axis angle by means of a phase difference of the P-polarized and S-polarized heterodyne signals via Hilbert transformation. However, a limited sampling rate in Hilbert transformation decreases detection sensitivity of the PSOCT.
In U.S. Patent Application Publication No. US 2008/0309946, the applicant disclosed a differential-phase interferometric system suitable for optical coherence tomography. The interferometric system includes a polarized heterodyne interferometer generating first and second optical heterodyne electrical signal outputs. A differential amplifier receives the first and second optical heterodyne electrical signal outputs from the polarized heterodyne interferometer, and generates a differential signal output. A data acquisition unit receives the first and second optical heterodyne electrical signal outputs from the polarized heterodyne interferometer and the differential signal output from the differential amplifier, and measures amplitudes of the first and second optical heterodyne electrical signal outputs and the differential signal output. A computing unit, such as a personal computer, is operable to compute the amplitudes measured by the data acquisition unit to determine a phase difference between the first and second optical heterodyne electrical signal outputs.
The polarized heterodyne interferometer includes a light source module for generating a circularly polarized output beam, a beam splitter for splitting the circularly polarized output beam into a reference beam and a signal beam, a piezoelectric-supported mirror for reflecting the reference beam, a first polarization beam splitter for splitting the signal beam into P-polarization and S-polarization wave components, a scanning mirror for reflecting the S-polarization wave component, a lens through which the P-polarization wave component passes for refection by an imaging plane in a specimen, a second polarization beam splitter for receiving a combined output beam of the reference and signal beams from the beam splitter and for separating the combined output beam into mutually orthogonal linear-polarized first and second optical signals, i.e., P-polarized optical signal and S-polarized optical signal, and first and second photo detectors for detecting the first and second optical signals, respectively, so as to generate the first and second optical heterodyne electrical signal outputs, respectively.
It is noted that, in the aforesaid patent publication, control of the piezoelectric-supported mirror and the scanning mirror of the polarized heterodyne interferometer for simultaneous movement is required in order to obtain a tomographic image of an imaging plane at a certain depth of the specimen.